1. Field of the Invention
The invention relates to a method of driving a plasma display panel, and more particularly to a method of driving an AC memory-operation type plasma display panel.
2. Description of the Related Art
A plasma display panel is structurally grouped into a DC (direct current) type panel having electrodes exposed to discharge gas, and an AC (alternating current) type panel having electrodes covered with a dielectric layer to prevent from being directly exposed to discharge gas. An AC type plasma display panel is further structurally grouped into a memory-operation type panel which operates by virtue of a memory function caused by a function of a dielectric layer to store electric charges therein, and a refresh-operation type panel which operates not using a memory function.
Hereinbelow are explained a structure of an AC memory-operation type plasma display panel and a method of driving the same.
FIG. 1 is a perspective broken view of a conventional AC type plasma display panel suggested in Japanese Patent Application Publication No. 2001-272948.
As illustrated in FIG. 1, a plasma display panel 20 includes an electrically insulating front substrate 1A and an electrically insulating rear substrate 1B.
On the front substrate 1A are arranged a scanning electrode 9 and a common electrode 10 spaced away from each other and in parallel with each other.
Each of the scanning electrode 9 and the common electrode 10 is comprised of a bus electrode 3 for presenting electrical conductivity, and a principal discharge electrode 2 formed on the bus electrode 3 for generating discharge therefrom. The principal discharge electrode 2 in the plasma display panel 20 is comprised of a transparent electrode composed of indium-tin oxide (ITO) or SnO2 for preventing reduction in light transmissivity.
The scanning electrode 9 and the common electrode 10 are covered with a dielectric layer 4a, which is covered with a protection film 5 composed of magnesium oxide to protect the dielectric layer 4a from discharges.
On the rear substrate 1B is arranged a plurality of data electrodes 6 extending in parallel with one another and perpendicularly to the scanning electrode 9 and the common electrode 10.
The data electrodes 6 are covered with a dielectric layer 4b. On the dielectric layer 4b is formed a plurality of partition walls 7 extending in parallel with the data electrodes 6 for defining discharge areas and display cells.
A phosphor layer 8 is formed on an exposed surface of the dielectric layer 4b and sidewalls of the partition walls 7 for converting ultra-violet rays generated by discharges, into visible light. By forming color phosphor layers in each of display cells, it would be possible to display colored images. For instance, color phosphor layers of three primary colors, that is, red (R), green (G) and blue (B) may be formed.
Discharge gas is introduced into a space sandwiched between the front and rear substrates 1A and 1B and partitioned by the partition walls 7. For instance, discharge gas is comprised of helium (He), neon (Ne) and xenon (Xe) alone or in combination.
FIG. 2 is a plan view of the plasma display panel 20 as viewed from a viewer.
As illustrated in FIG. 2, the scanning electrode 9 and the common electrode 10 extend in a row direction in parallel with each other. A gap formed between the scanning electrode 9 and the common electrode 10 is called a discharge gap 12, in which surface-discharge is generated between the scanning electrode 9 and the common electrode 10.
Hereinbelow, a method of driving the plasma display panel 20 is explained with reference to FIG. 3.
FIG. 3 is a timing chart showing waveforms of pulse voltages applied to the scanning electrode 9, the common electrode 10 and the data electrode 6, and further showing waveforms of a light emitted in normal operation and at generation of intensive discharge.
It is assumed in FIG. 3 that the previous sub-field is selected, but the illustrated sub-field is not selected.
Voltages are applied separately to each of the scanning and data electrodes 9 and 6, and voltages having a common waveform are applied to all of the common electrodes 10.
As illustrated in FIG. 3, a fundamental cycle for driving the plasma display panel 20 includes a reset period (A) in which display cells are reset for causing discharges to be readily generated in the subsequent period (B), a scanning period (B) in which it is selected which display cell or cells is(are) to be turned on or off, a sustaining period (C) in which discharges are generated in all of the selected display cells. Such a fundamental cycle is called a sub-field.
In the reset period, a sustaining-discharge eliminating pulse Pse is applied to all of the scanning electrodes 9 to generate charge-eliminating discharge to eliminate wall charges accumulated due to previous sustaining-discharge pulses.
Herein, the term “eliminate” should not be limited to elimination of all of wall charges, but should be interpreted as including reduction in wall charges for smoothly generating subsequent preliminary discharges, data-writing discharges and sustaining discharges.
The sustaining-discharge eliminating pulse Pse is a pulse voltage having an inclined waveform or a serrate waveform in which a voltage varies with the lapse of time.
Then, a positive priming pulse Pp+ is applied to all of the scanning electrodes 9 for causing compulsory discharges in all of the display cells. While the positive priming pulse Pp+ is being applied to the scanning electrode 9, a negative priming pulse Pp− is applied to the common electrodes 10.
Then, a priming-eliminating pulse Ppe is applied to all of the scanning electrodes 9 for causing charge-eliminating discharges to eliminate wall charges having been accumulated due to the positive priming pulse Pp+. The term “eliminate” should not be limited to elimination of all of wall charges, but should be interpreted as including reduction in wall charges for smoothly generating subsequent data-writing discharges and sustaining discharges.
Preliminary discharge caused by application of the positive priming pulse Pp+ and elimination of the preliminary discharge caused by application of the priming-eliminating pulse Ppe make subsequent data-writing discharge be readily generated.
Following the priming-eliminating pulse Ppe, a scanning base pulse Pbw is applied to the scanning electrode 9.
The positive priming pulse Pp+ and the priming-eliminating pulse Ppe have an inclined waveform or a serrate waveform in which a voltage raises or lowers with the lapse of time. Discharge generated by application of a voltage having such an inclined waveform is just weak discharge which can extend only in the vicinity of the discharge gap 12.
The above-mentioned preliminary discharge and charge-eliminating discharge are generated independently of images. Hence, light emission caused by those discharges is observed as background luminance. If the thus observed background luminance is at high level, contrast would be deteriorated, and hence, quality of images is degraded.
An operation of the plasma display panel 20 caused by the sustaining-discharge eliminating pulse Pse in a cross-section A1–A2 (see FIG. 2) of the data electrode 6 in a display cell is explained hereinbelow with reference to FIG. 4 and FIGS. 5A to 5E.
FIG. 4 illustrates the sustaining-discharge eliminating pulse Pse over a sustaining period to the next reset period, and FIGS. 5A to 5E illustrate wall charges in a reset period in the case that weak discharges are stably generated.
In a conventional method of driving the plasma display panel 20, a voltage Vs is applied to the scanning electrode 9, and the common electrode 10 is grounded at a final sustaining discharge in a sustaining period.
Thus, as illustrated in FIG. 5A, negative electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9 and positive electric charges are accumulated on the dielectric layer 4a above the common electrode 10 immediately before application of the sustaining-discharge eliminating pulse Pse and after sustaining discharge was generated. In contrast, positive electric charges are accumulated on the dielectric layer 4b above the data electrode 6, as illustrated in FIG. 5A.
During the application of the sustaining-discharge eliminating pulse Pse to the scanning electrode 9, the common electrode 10 is kept at the voltage Vs, and a voltage having an inclined or serrate waveform in which a voltage gradually varies to GND from the voltage Vs with the lapse of time is applied to the scanning electrode 9 (hereinbelow, such a voltage is referred to as “a serrate voltage”). After the application of the serrate voltage, when a sum of a voltage externally applied to the electrodes 9 and 10 and a voltage caused by wall charges exceeds a threshold voltage at which discharge starts, surface-discharge is generated between the scanning electrode 9 and the common electrode 10.
The surface electrode starts at a time Tfsw (see FIG. 4). If the serrate voltage has an inclination of about 10V/microsecond or smaller, the surface-discharge is generated as weak discharge gradually expanding as the serrate voltage varies, as illustrated in FIG. 5B.
As illustrated in FIG. 5C, weak discharge is generated between the scanning electrode 9 and the common electrode 10 further at a time Tfss (see FIG. 4).
When a sum of a voltage externally applied to the electrodes 9 and 6 and a voltage caused by wall charges exceeds a threshold voltage at which discharge starts, cross-discharge is generated between the scanning electrode 9 and the data electrode 6 wherein the data electrode 6 is at a positive voltage and the scanning electrode 9 is at a negative voltage. The cross-discharge starts at a time Tfm (see FIG. 4).
As shown in FIG. 4, the time Tfsw is earlier than the time Tfm at which the cross-discharge is generated between the scanning electrode 9 and the data electrode 6. That is, since the surface-discharge has been generated between the scanning electrode 9 and the common electrode 10, ions and metastables already exist in a discharge space, namely, the discharge space is already activated. Accordingly, the cross-discharge is stably generated between the scanning electrode 9 and the data electrode 6, as illustrated in FIG. 5D.
After the application of the sustaining-discharge eliminating pulse Pse to the scanning electrode 9, electric charges are accumulated as illustrated in FIG. 5E.
An operation of the plasma display panel 20 caused by the priming-eliminating pulse Ppe is explained hereinbelow with reference to FIG. 6 and FIGS. 7A to 7D.
FIG. 6 illustrates waveforms of the positive priming pulse Pp+ and the priming-eliminating pulse Ppe, and FIGS. 7A to 7D illustrate wall charges in a reset period.
While the positive priming pulse Pp+ having an inclined waveform is applied to the scanning electrode 9, the common electrode 10 is kept at GND.
When a sum of a voltage externally applied to the electrodes 9 and 10 and a voltage caused by wall charges exceeds a threshold voltage at which discharge starts, surface-discharge is generated between the scanning electrode 9 and the common electrode 10. The surface-discharge is generated as weak discharge gradually expanding as the serrate voltage varies, similarly to discharge generated by the application of the sustaining-discharge eliminating pulse Pse to the scanning electrode 9. The surface-discharge rearranges electric charges existing in the vicinity of the discharge gap 12.
At the same time, cross-discharge is generated between the scanning electrode 9 and the data electrode 6, resulting in that positive electric charges are accumulated on the dielectric layer 4b above the data electrode 6.
After the application of the positive priming pulse Pp+ to the scanning electrode 9 has been terminated, as illustrated in FIG. 7A, negative electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9, positive electric charges are accumulated on the dielectric layer 4a above the common electrode 10, and positive electric charges are accumulated on the dielectric layer 4b above the date electrode 6.
While the priming-eliminating pulse Ppe having a negatively inclined waveform is applied to the scanning electrode 9, the common electrode 10 is kept at the voltage Vs.
After the application of the priming-eliminating pulse Ppe to the scanning electrode 9, when a sum of a voltage externally applied to the electrodes 9 and 10 and a voltage caused by wall charges exceeds a threshold voltage at which discharge starts, surface-discharge is generated between the scanning electrode 9 and the common electrode 10. The surface-discharge starts at a time Tfsw (see FIG. 4). The surface-discharge is generated as weak discharge gradually expanding as the serrate voltage varies, as illustrated in FIG. 7B.
When a sum of a voltage externally applied to the electrodes 9 and 6 and a voltage caused by wall charges exceeds a threshold voltage at which discharge starts, cross-discharge is generated between the scanning electrode 9 and the data electrode 6. The cross-discharge starts at a time Tfm (see FIG. 4).
Weak discharge is generated between the scanning electrode 9 and the common electrode 10 also at a time Tfss (see FIG. 6)
The time Tfsw at which the surface-discharge is generated between the scanning electrode 9 and the common electrode 10 is earlier than the time Tfm at which the cross-discharge is generated between the scanning electrode 9 and the data electrode 6. That is, when the cross-discharge is generated between the scanning electrode 9 and the data electrode 6, the surface-discharge has been already generated between the scanning electrode 9 and the common electrode 10, as illustrated in FIGS. 7B and 7C.
After the application of the priming-eliminating pulse Ppe to the scanning electrode 9 has been terminated, electric charges are arranged such that operation in the subsequent scanning period can be smoothly carried out, as illustrated in FIG. 7D. That is, negative electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9, positive electric charges are accumulated on the dielectric layer 4a above the common electrode 10, and positive electric charges are accumulated on the dielectric layer 4b above the data electrode 6.
When not selected in the subsequent scanning period, that is, when data-writing discharge is not generated, wall charges are reduced to such a degree that discharge is not generated in a sustaining period.
In a scanning period in which discharge is generated to select a display cell in which a light is to be emitted, a scanning pulse Pw is applied to the scanning electrodes 9 one by one at different timings from one another, and a data pulse Pd having a voltage Vd is applied to the data electrode 6 in accordance with images to be displayed and in synchronization with a timing at which the scanning pulse was applied. The voltage Vd is equal to about 70V, for instance. In a display cell in which while the scanning pulse Pw is applied to scanning electrode 9, the data pulse Pd is applied to the data electrode 6, cross-discharge is generated between the scanning electrode 9 and the data electrode 6, and the cross-discharge induces surface-discharge to be generated between the scanning electrode 9 and the common electrode 10. A series of these actions is called data-writing discharge.
As a result of the generation of the data-writing discharge, positive electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9, negative electric charges are accumulated on the dielectric layer 4a above the common electrode 10, and negative electric charges are accumulated on the dielectric layer 4b above the data electrode 6.
As a result of first sustaining-discharge, negative electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9, and positive electric charges are accumulated on the dielectric layer 4a above the common electrode 10.
In a second sustaining-pulse, a voltage has a polarity opposite to a polarity of a voltage to be applied to the scanning electrode 9 and the common electrode 10 in accordance with a first sustaining-pulse. Hence, a voltage caused by electric charges accumulated on the dielectric layer 4a is added to a voltage in the second sustaining-pulse, and accordingly, there is generated second sustaining-discharge.
Hereinafter, sustaining-discharges are generated in the same way. If surface-discharge is not generated by virtue of the first sustaining-pulse, discharge will not be generated due to subsequent sustaining-pulses.
A combination of the above-mentioned reset period, scanning period and sustaining period is called a sub-field.
In order to accomplish displaying images at gray scales, one field which is a period for displaying one scene is divided into a plurality of sub-fields, and the different number of sustaining-pulses is assigned to each of sub-fields. If one field is divided into N sub-fields, and a luminance ratio among the sub-fields is defined equal to 2(N−1), it would be possible to display images at 2N gray scales by selecting sub-fields to be displayed in a field and combining them with one another.
For instance, it is assumed that one field is divided into eight (8) sub-fields. Since the eighth power of two is equal to 256 (28=256), it is possible to display images at 256 gray scales by controlling on/off of each of the eight sub-fields.
The above-mentioned conventional method of driving the plasma display panel 20 is accompanied with problems that weak discharge is not generated, but intensive discharge is generated at a voltage beyond a voltage at which weak discharge is to be generated, in a pulse having an inclined waveform in which a voltage gradually varies with the lapse of time, and that there is generated a difference in a panel in intensity of weak discharges, and resultingly, wall charges are not arranged uniformly in the panel.
FIG. 8 illustrates electric lines of force in an electric field generated between the scanning electrode 9 and the common electrode 10. The reason for the above-mentioned problems is explained hereinbelow with reference to FIG. 8.
As shown with electric lines of force in FIG. 8, an electric field generated between the scanning electrode 9 and the common electrode 10 is curved about the discharge gap 12 as a center. Hence, the electric filed has a relatively small density in an area remote from the discharge gap 12, whereas the electric field has a relatively high density in an area close to the discharge gap 12. Accordingly, a remarkably intensive electric field is generated at the discharge gap 12.
FIGS. 9A to 9E illustrate arrangement of wall charges in a reset period in the case that there is generated intensive discharge.
In the conventional method of driving the plasma display panel 20, the voltage Vs is applied to the scanning electrode 9, and the common electrode 10 is kept at GND when final sustaining-discharge is generated in a sustaining period.
Accordingly, after the generation of the sustaining-discharge has been terminated and immediately before the sustaining-discharge eliminating pulse Pse is applied to the scanning electrode 9, negative electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9, positive electric charges are accumulated on the dielectric layer 4a above the common electrode 10, and positive electric charges are accumulated on the dielectric layer 4b above the data electrode 6, as illustrated in FIG. 9A.
If an efficiency at which discharge is generated is lowered at the application of the sustaining-discharge eliminating pulse Pse, surface-discharge is not accidentally generated at the time Tfsw (see FIG. 9B), but is sometimes generated at a time later than the time Tfsw.
If surface-discharge is generated between the scanning electrode 9 and the common electrode 10 at a time later than the time Tfsw, a voltage difference higher than a voltage difference found at a time at which discharge should start is applied across the scanning electrode 9 and the common electrode 10, because a voltage in a pulse having an inclined waveform is lowered during the time Tfsw to the time at which surface-discharge is actually generated. As a result, the resultant surface-discharge expands to a higher degree than weak discharge, that is, there is generated discharge slightly more intensive than expected.
As mentioned above, a remarkably intensive electric field is generated at the discharge gap 12 formed between the scanning electrode 9 and the common electrode 10. Hence, if there is generated discharge slightly more intensive than expected, the discharge swiftly grows into intensive discharge which expands all over a display cell, as illustrated in FIG. 9C.
The time Tfss shown in FIG. 4 is an earliest time at which such intensive discharge may be generated.
If intensive discharge is generated, positive electric charges are accumulated entirely on the dielectric layer 4a above the scanning electrode 9, and negative electric charges are accumulated entirely on the dielectric layer 4a above the common electrode 10, as illustrated in FIG. 9D.
Hereinafter, since discharge is never generated during application of a pulse voltage having an inclined waveform, to the scanning electrode 9, wall charges are arranged as illustrated in FIG. 9E after the application of the sustaining-discharge eliminating pulse Pse. That is, positive electric charges are accumulated on the dielectric layer 4b above the data electrode 6, but positive electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9, and negative electric charges are accumulated on the dielectric layer 4a above the common electrode 10, contrary to the arrangement of wall charges illustrated in FIG. 5E.
After the application of the sustaining-discharge eliminating pulse Pse to the scanning electrode 9, wall charges are re-arranged by the positive priming pulse Pp+ and the priming-eliminating pulse Ppe. Arrangement of wall charges by the pulses Pp+ and Ppe is accomplished by generating weak discharge, similarly to the sustaining-discharge eliminating pulse Pse. Hence, influence caused by intensive discharge generated when the sustaining-discharge eliminating pulse Pse is applied to the scanning electrode 9 can be eliminated in the vicinity of the discharge gap 12. However, it will be impossible to eliminate such influence all over a display cell. In particular, in an area remote from the discharge gap 12, positive electric charges remain accumulated on the dielectric layer 4a above the scanning electrode 9, and negative electric charges remain accumulated on the dielectric layer 4a above the common electrode 10.
In a subsequent scanning period, voltages applied to the electrodes 9 and 10 are determined such that the plasma display panel can stably operate when negative electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9, and positive electric charges are accumulated on the dielectric layer 4a above the common electrode 10 (see FIG. 5E). Accordingly, if positive electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9, and negative electric charges are accumulated on the dielectric layer 4a above the common electrode 10, the plasma display panel operates unstably.
In order to reduce a background luminance, the positive priming pulse Pp+ and the priming-eliminating pulse Ppe are not sometimes applied to the scanning electrode 9 in a certain sub-field. This is because it is possible to arrange wall charges similarly to the arrangement of wall charges found after the application of the priming-eliminating pulse Ppe, even after wall charges have been arranged by the sustaining-discharge eliminating pulse Pse. Hence, the plasma display panel can operate stably in a subsequent scanning period in the same way as a case in which the positive priming pulse Pp+ and the priming-eliminating pulse Ppe are applied to the scanning electrode 9.
However, if there is generated intensive discharge in the sustaining-discharge eliminating pulse Pse, positive electric charges are accumulated on the dielectric layer 4a above the scanning electrode 9, and negative electric charges are accumulated on the dielectric layer 4a above the common electrode 10, as illustrated in FIG. 9E. Since a subsequent scanning period starts in such a condition, there is caused erroneous light-emission, that is, light is emitted even in a non-selected display cell.
In addition, if positive electric charges accumulated on the dielectric layer 4a above the scanning electrode 9 and negative electric charges accumulated on the dielectric layer 4a above the common electrode 10 are not sufficiently eliminated, there is generated intensive discharge 30B (see FIG. 3) as erroneous discharge in a sustaining period, or the priming-eliminating pulse Ppe causes intensive discharge, and accordingly, there is generated intensive discharge 30B (see FIG. 3) as erroneous discharge in a sustaining period.
In order to prevent such erroneous light-emission, it is necessary to prevent generation of intensive discharge in the sustaining-discharge eliminating pulse Pse. If it is not possible to prevent generation of such intensive discharge, it would be necessary to prepare a countermeasure to such intensive discharge.
If an efficiency at which discharges are generated in the priming-eliminating pulse Ppe, similarly to the sustaining-discharge eliminating pulse Pse, weak discharge may not be generated between the scanning electrode 9 and the common electrode 10.
If discharge is generated later, the resultant discharge would be more intensive than weak discharge because of a higher voltage difference than a voltage difference found at a time at which discharge should start is applied across the scanning electrode 9 and the common electrode 10. Since a remarkably intensive electric field is generated at the discharge gap 12 formed between the scanning electrode 9 and the common electrode 10, the discharge swiftly grows into intensive discharge 30A (see FIG. 3) which expands all over a display cell. The time Tfss shown in FIG. 6 is an earliest time at which such intensive discharge 30A is generated.
The generation of intensive discharge results in that positive electric charges are accumulated entirely on the dielectric layer 4a above the scanning electrode 9, and negative electric charges are accumulated entirely on the dielectric layer 4a above the common electrode 10. This is the same arrangement of wall charges as the arrangement found after data-writing discharge has been generated in a selected display cell in a scanning period.
Accordingly, even if not selected in a subsequent scanning period, if intensive discharge 30A is generated in the priming-eliminating pulse Ppe, there would be generated discharge because of addition of wall charges to an externally applied voltage when a first sustaining-pulse Ps is applied to the electrodes. Discharges are continuously generated even in second and later sustaining pulses Pse.
As a result, there is caused erroneous light-emission, that is, light is emitted even in a non-selected display cell. In order to prevent such erroneous light-emission, it would be necessary to prevent generation of the intensive discharge 30A in the priming-eliminating pulse Ppe, or to eliminate influence exerted by the intensive discharge 30A, even if the intensive discharge 30A was generated.
As explained above, the conventional method of driving the plasma display panel 20 is accompanied with a problem that images are deteriorated as a result that light is emitted in a non-selected display cell, namely, there occurs erroneous light-emission.
For instance, Japanese Patent Application Publication 2000-122602 has suggested a method of driving a plasma display panel which method is capable of solving a problem of erroneous light-emission.
Specifically, in the suggested method, surface-discharge and cross-discharge in charge-eliminating discharge are generated temporally separately from each other.
However, the suggested method is accompanied with a problem that if the discharges are concurrently generated, it would be quite difficult to control electric charges accumulated above a data electrode with the result of erroneous operation in a scanning period.
Specifically, if a ratio at which discharges are generated is quite low, priming particles are soon reduced when a certain period of time passes after generation of discharge. Accordingly, if surface-discharge and cross-discharge are generated temporally separately from each other as in the above-mentioned method, even if cross-discharge is first generated as weak discharge, subsequent surface-discharge will be generated as intensive discharge.
Thus, even in the above-mentioned method, the problem that light is emitted in a non-selected cell due to intensive discharge is not always solved.